1. Field of the Invention
The invention relates to apparatus for making paper, and in particular to a wet part or portion of a paper making machine.
2. Description of Related Technology
It is known to provide a wet part of a paper making machine with a distributor to which a main stream of a stock suspension is fed and which has a number of connections. The connections are connected by means of split stream lines to a machine-wide flow box which has a machine-wide slice nozzle (headbox). The split stream lines are connected (directly or indirectly) to the flow box. The slice nozzle is connected to at least one continuous web-forming screen. At least one collection trough is connected to the screen, as well as, optionally, to a backwater tank to accept the backwater trapped by the collection trough. Dilution water lines are provided to introduce backwater into the split stream lines to adjust the stock density of the split streams.
A particularly crucial component of such a wet part of a paper making machine is the flow box with the slice nozzle (the so-called headbox). This decisively determines the quality of the produced paper web.
The most important quality requirements concern fiber orientation in the pulp stream leaving the slice nozzle and in the paper web after its production, as well as the basis weight profile, i.e., the distribution of basis weight both lengthwise and across the machine direction of the paper web.
It has been possible, based on new developments, to bring the basis weight transverse profile under control in an outstanding fashion. This occurs by introducing dilution water into the split streams coming from the distributor.
Backwater, i.e., the water separated from the pulp during the sheet formation process, is preferably used as dilution water. This backwater is trapped in a collection trough, the so-called machine pit, from which it enters into a backwater tank of a relatively high volume. Use of backwater is more favorable for a variety of reasons than use of fresh water. It saves on fresh water. In addition, the valuable fibers and fillers contained in the backwater are reused. Connection lines are provided that make a connection between the backwater tank and the partial (split stream) flow lines to the flow box.
Maintaining a constant basis weight profile over the width of the paper web is possible based on the said modern development. However, it has been shown that the basis weight of the paper web does not remain constant over longer periods even with this principle of basis weight cross profile control. The reasons correspond essentially to those of conventional cross profile control. For example, the paper web deviates in basis weight with respect to hours of production. What changes then is the longitudinal (lengthwise or machine direction) profile of basis weight. The cross profile (transverse to the lengthwise or machine direction) can remain essentially constant.
There has been no lack of effort to determine the cause of this lack of constant basis weight profile and to overcome it. Thus, an attempt was made to solve the problem by means of control and supplying varying amounts of pulp and filler into the split stream lines during deviations. However, this was unsuccessful, insofar as the deviations in basis weight in the longitudinal direction had already occurred by the time they were recognized, so that the controlling intervention by appropriate supply of dilution water came too late. The period between occurrence of the disturbance and the corrective intervention is generally significant, since recording of reliable measured values is very time-intensive.